Originally Posted by
torger
Here's the size 54 and size 56 compared (link below). Press "Shadow Bike" to enable a dashed shadow, and "Swap Bikes" to swap between the 54 and 56, and edit saddle height, spacers and stem length as you like.
The 56 can be made a little bit less aggressive. Between 54 and 56 you often get a bit more stack than reach. The seat tube angle is laid back though which should push the saddle further back and add more reach, assuming you have a higher saddle on the larger bike. But as we fit the same person the saddle height is the same and the saddle isn't moved as far back. We still need to push it a little bit forward and there's still a few mm longer reach left on the 56.
Anyway, sizing up is sometimes better than sizing down when you want a more relaxed fit. It depends a bit on how many spacers you can fit under the stem.
https://www.bikegeocalc.com/#22Canno...5F172.5G30H30Z
You can't directly compare the Reach on two bikes with different Stack. Add a spacer to the smaller bike to equalize Stack and you'll find the Reach is greater than the numbers say it is. That's because the head tube makes the Reach 3mm shorter for every 10mm in increased Stack.
The real difference in reach from a given saddle set back to a particular stem height is 11mm.
Seat tube angles don't dictate your saddle set back.